ABSTRACT: We investigated the partitioning of carbon, nitrogen, and phosphorus between particulate and dissolved production using 11-m3 marine mesocosms (bags) in a Norwegian fjord with a salinity of 28.3, a chlorophyll concentration of 0.6 µg L-1, an even biomass among five algal groups, and nitrogen limitation as the initial conditions. The experiment lasted 21 days in August. Addition of silicate (+Si) resulted in diatom dominance, while a more diverse community was present in treatments with no added Si (-Si). Addition of inorganic nutrients in a N: P gradient from 64 to 4 either conserved the initial N limitation or forced the plankton communities to P limitation. Per added limiting nutrient, the diatom-dominated bags produced more particulate (POC) and dissolved organic carbon (DOC) than the other bags. However, the relative partitioning of net production to POC and DOC did not differ as a function of the plankton communities. Between 22% and 33% of the net production accumulated as new DOC. The higher values were found in the N-limited bags. The production of new dissolved organic nitrogen (DON) was variable over time, and short periods of positive production were followed by removal (negative production). Between 6% and 22% of the assimilated N was recovered as new DON in the N-replete bags, while the DON production was very low during N limitation. The community structure had no effects on nitrogen partitioning. Diatom dominance (+Si bags) resulted in P sequestration to particles and a constant low net production of dissolved organic phosphorus (DOP) across the nutrient gradient. The production of DOP was low in the P-limited (-Si) bags; however, with a surplus of inorganic P, most of the assimilated P (74% to 85%) was recovered as new DOP. The consequence was a huge range in stoichiometric ratios for newly produced dissolved organic matter (DOM). With N limitation, the C:N ratio of new DOM was from 40 to 100, but it was below 40 under N-replete conditions. The C: P ratio of new DOM in the -Si bags traced the P availability, and values approached 500 in P-deficient bags to values between 17 and 58 in the P-replete bags. The C: P ratio of new DOM in the +Si bags was about 300 at all dosing regimes. Consequently, the range in N: P ratios was also large, with values from below 1 to about 30. Carbon-rich DOM in oceans and coastal waters is not necessarily a function of a slow diagenetic maturation process but can be produced almost immediately. Both the nutrient regime and phytoplankton community composition affected the production and composition of new DOM in this experiment.

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